The Apocalypse is the biblical event referring to, quite literally, a series of events that will eventually lead to the end of the world.

The Apocalypse was foretold thousands of years ago, shortly after Lucifer was imprisoned in The Cage. The prophecy stated that the 66 Seals would be broken and that Lucifer would escape the Cage, bringing about the Apocalypse.

The Four Horsemen of the Apocalypse would assist him in destroying humanity. God's eldest Archangel Michael was to then take a human vessel descended from Cain and Abel and lead the "final charge" against Hell and personally kill Lucifer in battle. Lucifer's death would bring about Paradise on Earth (or what was left of it).

According to Gabriel, the Apocalypse isn't a war, but a fight between two brothers "who loved each other, and betrayed each other".

In his quest to destroy humanity, Lucifer called upon the Four Horsemen of the Apocalypse. War was used to turn humanity against itself, Famine was sent to exploit their needs, and Pestilence was employed to ultimately exterminate them through the Croatoan Virus. The Winchesters, assisted by various allies, managed to foil these Horsemen's plans and steal their rings of power.

Jesse Turner, a boy born from a woman possessed by a demon, was meant to become the Anti-Christ, destined to help Lucifer not only destroy humanity, but also destroy the Host of Heaven. Lucifer's presence on Earth increased his powers tremendously. Even though he was still a child, he was powerful enough to turn Castiel into a toy figure. However, thanks to Sam and Dean's encouragement, Jesse rejected his destiny and went into hiding.

The Whore of Babylon rose sometime after Lucifer and assumed the guise of a Pastor's daughter, Leah. As Leah, she pretended to be a Prophet of God. Her purpose was to damn all she could to Hell by manipulating people into shedding innocent blood in God's name. She was stopped when Sam, Dean, and Castiel discovered her true identity, and Dean stabbed her with a Stake made from a Cypress tree.

Death, the oldest and most powerful Horseman, also served Lucifer, but did so unwillingly and resented being bound to a "bratty child throwing a temper tantrum." Death eventually met with Dean and gave him his ring, which the Winchesters needed to re-imprison Lucifer.

In 1972, after years of searching, the demon Azazel found the entrance to Lucifer's Cage at St. Mary's Convent. He contacted Lucifer, who told him of his plans: Azazel was to free Lilith, the first demon, from Hell so that she could break the 66 Seals. In addition, Azazel was also tasked with finding a special child strong enough to kill Lilith, thereby breaking the Final Seal, as well capable of serving as Lucifer's Vessel for the final battle against Michael. In 2007, Azazel succeeded in both freeing Lilith and finding Lucifer's special child, Sam Winchester, the younger brother of Dean Winchester, Michael's true Vessel.

A year later, Dean Winchester, who had sold his soul to save Sam, was taken to Hell. There he was tortured mercilessly by the demon Alastair. The forces of Heaven laid siege to Hell and eventually broke through their defenses. The angel Castiel personally pulled Dean out of Hell. By then, however, Dean had broke and started torturing souls himself.

Soon after Dean's resurrection, hunters are attacked by the ghosts of people they could not save. Bobby's research uncovers an obscure translation of the Book of Revelation that lists the ghosts, or the Witnesses, as a sign of the upcoming Apocalypse. Castiel confirms that demons, under Lilith, are attempting to break the 66 Seals and free Lucifer from his cage which will bring the Apocalypse. The Rising of the Witnesses was one of these seals.

The Winchesters, Bobby, and Angels fight to stop other seals from breaking, but they steadily lose ground. There are over 600 Seals, and only 66 need to be broken. They discover that the First Seal was broken by Dean in Hell, when he became a torturer. Sam feels compelled to kill Lilith, who he has been told is the only being that can break the Final Seal, while the Angels tell Dean that it is he who must prevent the Apocalypse.

The Final Seal is broken.

In fact, both brothers are being manipulated by their "allies." Ruby is secretly an agent of Lucifer, working to get Sam to use his special demon powers. The higher ranking angels including Michael, Raphael and Zachariah want the Apocalypse to happen so that they can draw Lucifer out into the open and kill him. The Angels hold Dean in the Green Room while Sam and Ruby track Lilith down to St. Mary's Convent where the Final Seal is to be broken. Castiel frees Dean and sends him after Sam, but Sam uses his powers to kill Lilith, whose death is actually the Final Seal. Lucifer's Cage opens, and the Apocalypse begins.

As Lucifer rises from his Cage, the brothers are teleported out of harm's way by God. Finding themselves on an airplane, they witness Lucifer burst out of his prison in a blast of white light.

Immediately, Sam and Dean hear of earthquakes, tornadoes and disease spreading. Zachariah confronts the brothers, and reveals that the real carnage will not start until Lucifer enters his preordained vessel. Zachariah also reveals that Dean alone cannot possibly kill Lucifer, but he tells Dean that he is Michael's vessel, and once occupied, he can defeat Lucifer and bring paradise to Earth - though half the planet will also be destroyed. Sam learns later, from the Devil himself, that he is Lucifer's true vessel. Neither brother is willing to consent to serve as a vessel, postponing the final battle. Michael remains disembodied, but Lucifer finds a temporary vessel in Nick.

The Beginning of the End.

Most demons fall in line behind Lucifer, and his plans proceed. He intends to destroy humanity, and essential to his goals are the Four Horsemen of the Apocalypse. War is the first encountered by Sam and Dean. He causes an entire town to believe there are demons among them when there are none. Famine, a weakened but still intimidating figure, is cared for by demons at Lucifer's direction. Pestilence, again assisted by demons, devises a scheme to spread the Croatoan Virus as fast as possible. Death, the oldest and most powerful horseman, is freed and Lucifer finds a way to force his obedience. With Death under his control, Lucifer orders still greater disruption. The Whore Of Babylon also serves as an instrument of destruction and misinformation. Jesse Turner, the Anti-Christ, was supposed to play an important role during the destruction of Earth, but he decided to turn away from Lucifer's plans.

Meanwhile, Castiel, having been resurrected by God, goes on a search to find God, believing Him to be capable of putting an end to the Apocalypse. His efforts however are futile, as even the Archangels themselves believe God to be dead or having abandoned them.

Sam and Dean follow omens, from forest fires to biblical storms to the dead rising, and combat what they find. Other hunters are hostile toward them because rumor has spread of their involvement in starting the Apocalypse. They are constantly assaulted by both angels and demons who want to force them to serve as vessels so that the last battle may take place; all the entities against them are very concerned with destiny. Even old allies turn against them. Anna Milton, the fallen angel who helped them in the previous season, attempts to stop the Apocalypse on her own by going back in time and preventing Sam's birth. She is stopped by the Winchesters and the Archangel Michael.

Dean shoots Lucifer with the Colt. It doesn't work as well as he hoped...

The Winchesters find an unlikely ally in the Crossroads Demon Crowley, who gives them the Colt to kill Lucifer. With the aid of Castiel, Ellen, and Jo, the Winchesters launch an attack on Lucifer. Unfortunately, the Colt fails to kill Lucifer, and Ellen and Jo are lost in the process. The Winchesters are then dealt a further blow when they learn that God no longer cares about the Apocalypse and will not step in to help.

Dean, slowly worn down by the losses and defeats, and afraid that Sam will give in to Lucifer, and that Lucifer will win, decides to say yes to Michael, but the efforts of Sam, Bobby, and Castiel keep him away from the archangel until he decides to keep fighting. After Dean rejects Michael one final time and kills Zachariah, Michael takes Adam, Sam and Dean's half-brother, as his host.

The lost Archangel Gabriel allies himself with the Winchesters and tries to kill Lucifer, but is killed himself. However, he manages to leave the Winchesters a message that tells them how they can re-imprison Lucifer using the rings of the Four Horsemen. Having already acquired Famine and War's rings, Sam, Dean, and their allies do whatever is necessary to acquire the rings from Pestilence and Death. With Crowley's help, they track down Pestilence and steal his ring. Death, seeking to be free from Lucifer, willingly gives Dean his ring.

Sam (Lucifer) and Adam (Michael) fall into Lucifer's Cage together.

Sam comes up with a plan to say "yes" to Lucifer, so that the angel will enter him and then Sam will throw himself into the pit. Team Free Will is reluctant to agree, but does so. Sam takes Lucifer in, but cannot control him.

Lucifer later takes Sam to Stull Cemetery to confront Michael, who now has Adam. Dean, Castiel, and Bobby interrupt them, however, and Castiel succeeds in temporarily banishing Michael. Lucifer kills Castiel and Bobby, but Dean manages to reach Sam.

Now in control, Sam opens the Cage and prepares to fall in. Michael tries to stop him, but Sam grabs him by the arm and pulls all four of them into the Cage, ending the Apocalypse. who opens the portal and tips himself, Lucifer, Adam, and Michael, over the edge and into the Cage.

God resurrects Castiel, again, this time as a Seraphim. Castiel heals Dean and resurrects Bobby before taking off to take control of Heaven in Michael's absence. Dean and Bobby later part ways.

After the Apocalypse, a civil war broke out in Heaven. Raphael, the leader of the traditionalists, intended to free Michael and Lucifer so that the Apocalypse would restart and the predestined end of the world could still be reached. Castiel, who was resurrected by God once again, opposed him, and managed -- with the help of Crowley -- to destroy the Archangel, inadvertently releasing a new threat upon the world in the process.

The angel Metatron revealed to Castiel that with the end of the Apocalypse and the fall of the Archangels, Heaven was in chaos with a new civil war waging between many factions trying to take control; Naomi was revealed to be the leader of one of these said factions.

After the release of the Darkness, Sam visits Lucifer in Hell in hopes of learning how to defeat the Darkness. Lucifer claims to be able to defeat the Darkness if he is released, but Sam ultimately refuses as he believes Lucifer will restart the Apocalypse once the Darkness is gone. Lucifer eventually manages to escape his Cage once again by taking Castiel as his vessel. Lucifer also tells Sam that being trapped in the Cage has driven Michael insane, further confirmed by God saying that Michael is in no shape to join them in battle against the Darkness months later.

The term 'Armageddon' refers to the climatic duel between Jesus Christ and the Anti-Christ. And the term 'Judgment Day' refers to a time frame when all people stand before God the Father and are judged.

Incidentally, according to Death, at the End of all things, he'll reap God.

The term "Apocalypse", in Greek, actually means "Revelation" which is why the last book of The Bible is called "Revelation", given that it was John's revelation of the End from God. Many New-Age movements suggest a less cataclysmic event of the "Unveiling" or the "Revelations" and more of an access of hidden knowledge, or humanity gaining a higher power or plane of existence.